1. Technical Field
The present disclosure relates to a gold/titanium nanotube-multiwalled carbon nanotube composite, a method of making the gold/titanium nanotube-multiwalled carbon nanotube composite, and a method for the oxidation of cyclohexane wherein the nanotube composite is a catalyst in the oxidation.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Titanium oxide nanotubes (TiO2 nanotube, TNT) are nanostructured oxides having tubular shapes with almost no absorption in the visible light region (S. Iijima, Nature 354 (1991) 56-58—herein incorporated by reference in its entirety). UV light-sensitive chemical reactions occur at the surface of the nanotube (N. G. Chopra, R. J. Luyken, K. Cherrey, V. H. Crespi, M. L. Cohen, S. G. Louie, A. Zettl, Science 269 (1995) 966-968—herein incorporated by reference in its entirety). Researchers have fabricated a variety of geometric structures of TiO2 nanoparticles including zero dimensional (0D) structures such as spheric nanoparticle (Y. Feldman, E. Wasserman, D. J. Srolovitz, R. Tenne, Science 267 (1995) 222-225; M. E. Spahr, P. Bitterli, R. Nesper, M. Müler, F. Krumeich, H. U. Nissen, Angew. Chem. Int. Ed. 37 (1998) 1263-1265—each incorporated herein by reference in its entirety), and one-dimensional (1D) structures such as nanowires (P. M. Ajayan, O. Stephan, Ph. Redlich, C. Colliex, Nature 375 (1995) 564-567; B. C. Satishkumar, A. Govindaraj, E. M. Vogl, L. Basumallick, C. N. R. Rao, J. Mater. Res. 12(3), (1997) 604-606—each incorporated herein by reference in its entirety), nanorods (H. Nakamura, Y. Matsui, J. Am. Chem. Soc. 117(9), (1995) 2651-2652—incorporated herein by reference in its entirety), nanobelts (P. Hoyer, Langmuir 12 (1996) 1411-1413; T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14 (1998) 3160-3163—each incorporated herein by reference in its entirety) or nanotubes (T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11 (1999) 1307-1311; D. Gong, C. A. Grimes, O. K. Varghese, W. Hu, R. S. Singh, Z. Chen, E. C. Dickey, J. Mater. Res. 16 (2001) 3331-3334—each incorporated herein by reference in its entirety). The titanium oxide nanotube is mainly useful in dealing with waste-water due to the large surface to volume ratio. This improves the photocatalytic activity compared with spherical particles under UV-Vis irradiation. Numerous efforts have been developed to fabricate TiO2 nanoscale materials with special morphologies by employing traditional methods such as, sol-gel, micelle, and hydrothermal or solvothermal methods (H. Masuda, K. Nishio, N. Baba, Jpn. J. Appl. Phys. 31 (1992) L1775-L1777; P. Hoyer, Langmuir 12 (1996) 1411-1413—each incorporated herein by reference in its entirety). In general, one main drawback of the TiO2 nanostructures, when used in the practical application, comes from their easy loss during the process of water treatment. This results in low utilization rate and high cost, which limits their widespread use. Some attempts have been employed to improve the reuse efficiency of TiO2 via immobilization onto some supports such as carbon nanotube (B. O'Regan, M. Grätzel, Nature 353 (1991) 737-739; S. Hasegawa, Y. Sasaki, S. Matsuhara, Sens. Actuator B 13-14 (1993) 509-510—each incorporated herein by reference in its entirety), glass (G. H. Du, Q. Chen, R. C. Che, Z. Y. Yuan, L. M. Peng, Appl. Phys. Lett. 79 (2001) 3702-3704—incorporated herein by reference in its entirety), ceramic (Q. H. Zhang, L. A. Gao, J. Sun, S. Zheng, Chem. Lett. 31 (2002) 226-227—incorporated herein by reference in its entirety), and activated carbon (S. X. Liu, X. Y. Chen, X. Chen, Journal of Hazardous Material, vol. 143 (2007) 257-263; T. T. Lim, P. S. Yap, M. Srinivasan, A. Fane, Critical Reviews in Environmental Science and Technology, 41 (2011) 1173-1230; R. Kumar, S. K. Sithambaram, S. L. Suib, Journal Catalysis 262 (2009) 304-313—each incorporated herein by reference in its entirety). However, the current immobilization techniques are still not stable enough to improve the reaction efficiency due to decreasing TiO2 dispersion as well as its leaching from the supports. Among the previous supports, nanocarbon particles were chosen because they may increase remarkably the photoactivity of TiO2. Use of nanocarbon materials was found to provide attractive properties including exceptional electronic, adsorption, chemical inertness and stability (H. Slimen, A. Houas, J. P. Nogier, Journal of Photochemistry photobiology A: Chemistry, 221 (2011) 13-21; Y. Suzuki, S. Yoshikawa, J. Mater. Res. 19 (2004) 982-985; B. Seger, P. V. Kamat, J. Phys. Chem. C 113 (2009) 7990-7995; K. Woan, G. Pyrgiotakis, W. Sigmund, Advanced Materials 21(2009) 2233-2239—each incorporated herein by reference in its entirety). In recent studies (G. Socol, Yu. Gnatyuk, N. Stefan, N. Smirnova, V. Djokic', C. Sutan, V. Malinovschi, A. Stanculescu, O. Korduban, I. N. Mihailescu, Thin Solid Films 518 (2010) 4648-4653; R. Leary, A. Westwood, Carbon 49 (2011) 741-772; K. A. Wepasnick, B. A. Smith, K. E. Schrote, H. K. Wilson, S. R. Diegelman, D. H. Fairbrother, Carbon 49 (2011)24-36—each incorporated herein by reference in its entirety), attention was paid to the fact that TiO2 is an n-type semiconductor and the major process in photo-catalysis is activated by photon absorption and electron—hole formation. An enhancement of the photocatalytic properties of TiO2 can be accomplished via functionalization with CNT that hinders electron—hole pair recombination. In this way, TiO2 effectively behaves as a p-type semiconductor in the TiO2/CNT nanocomposites. Additionally, the large number of active adsorption sites at the catalyst surface and the improved suppression of the recombination of the charge carriers contribute to increasing photocatalytic activity. Many specific methods for the synthesis of TiO2/CNT nanocomposites have been developed, which generally consist of two steps: functionalization of the CNT and the nanocomposite synthesis. One of the functionalization methods applied to CNT is oxidative treatment, upon which the nanotubes become shortened, less tangled with ends opened, and oxygen-containing functional groups are introduced on their surfaces. The mentioned groups have a pronounced effect on the surface properties of the carbonaceous material, providing numerous sites for TiO2 bonding (J. P. Chen, S. Wu, Langmuir 20 (2004) 2233-2242; A. J. Plomp, D. S. Su, K. P. deJong, J. H. Bitter, Journal of Physical Chemistry C 113 (2009) 9865-9869; H. F. Gorgulho, J. P. Mesquita, F. Gonc-alves, M. F. R. Pereira, J. L. Figueiredo, Carbon 46 (2008) 1544-1555—each incorporated herein by reference in its entirety). The preparation and use of a TiO2/nanocarbon composite photocatalyst by coating anatase TiO2 having a nanospindle structure onto the oxidized surface of multi-wall carbon nanotubes (MWCNT) and the use of the resulting composite for photocatalytic cyclohexane oxidation was described.
Assembling metal particles either in the wall of titania and/or in carbon nanotubes or both will enhance their photoelectricity, electromagnetism and catalytic properties. Gold catalysts have been successfully used for cyclohexane oxidation based on various supports including metal oxides (M. Conte, X. Liu, D. M. Murphy, K. Whiston, G. J. Hutchings, Phys. Chem. Chem. Phys. 14 (2012) 16279-16285—incorporated herein by reference in its entirety), metal organic frameworks, (Z. G. Sun, G. Li, L. P. Liu, H. O. Liu, Catal. Commun. 27 (2012) 200-205—incorporated herein by reference in its entirety) mesoporous silica (L.-X. Xu, C.-H. He, M.-Q. Zhu, K.-J. Wu, Y.-L. Lai, Catal. Commun. 9 (2008) 816-820; X. Jiang, H. Deng, X. Wang, Colloid Surf. A: Physicochem. Eng. 358 (2010) 122-127; J. Xie, Y. Wang, Y. Li, Y. Wei, React. Kinet. Mech. Catal. 102 (2011) 143-154—each incorporated herein by reference in its entirety) based materials, and hydroxyapatite (Y. Liu, H. Tsunoyama, T. Akita, S. Xie, T. Tsukuda, ACS Catal. 1 (2011) 2-6—incorporated herein by reference in its entirety). It has been questioned whether gold catalysts actually do act as catalysts or as promotors of the autoxidation reaction (C. Della Pina, E. Falletta, M. Rossi, Chem. Soc. Rev. 41 (2012) 350-369—incorporated herein by reference in its entirety). Some authors support that gold acts as a catalyst for this reaction (A. Alshammari, A. Koeckritz, V. N. Kalevaru, A. Bagabas, A. Martin, Chem-Cat Chem 4 (2012) 1330-1336—incorporated herein by reference in its entirety) whereas others concluded that it works via a pure radical pathway with products typical of autoxidation (B. P. C. Hereijgers, B. M. Weckhuysen, J. Catal. 270 (2010) 16-25—incorporated herein by reference in its entirety). In general, Au/oxide studies were performed at high temperatures (above 373 K) and at oxygen pressures ranging from 0.3 to 3 MPa, with no solvent. On the other hand, titanium based catalysts like titanium silicalite-1 and Ti-MCM-41 when used with H2O2 presented low conversion for cyclohexane (E. V. Spinace, H. O. Pastore, U. Schuchardt, J. Catal. 157 (1995) 631-639; W. A. Carvalho, P. B. Varaldo, M. Wallau, U. Schuchardt, Zeolites 18 (1997) 408-412—each incorporated herein by reference in its entirety). V- and Cr-MCM-41 gave higher activities compared to Ti-MCM-41, however all metallosilicates undergo leaching of the metal and the observed catalytic activity was mainly due to the leached homogeneous metal species. Although H2O2 is environmentally friendly since it produces water as the only by-product, the major disadvantage of H2O2 is the instability that permits its decomposition into oxygen and water, a process which is accelerated in the presence of transition metal complexes and metal oxides (G. Strukul, Catalytic Oxidations with Hydrogen Peroxide as Oxidant, Kluwer Academic Publishers, Dordrecht, 1992—incorporated herein by reference in its entirety). The disadvantages like low conversion, leaching of metal, and over-oxidation products in existing catalytic systems have led researchers to find a catalyst that can improve the conversion of cyclohexane under ambient conditions, with high selectivity to cycloexanone/cyclohexanol ratio.
In view of finding a way for overcoming the above-mentioned obstacles, titanium nanotube photoatalysts synthesized by hydrothermal method incorporated with MWCNT and loaded with gold are tested in the selective photo-oxidation of cyclohexane, for evaluating the effect of surface modification as well as morphology on the catalysts' behavior. This is attained via using the versatile and green oxidant H2O2 to avoid using traditionally oxidizing agent; such as CrO42−, MnO4− and ClO4−, that produce toxic by-products.
The present disclosure describes a gold loaded TNT and TNT-MWCNT composition. Gold loaded TNT and TNT-MWCNT is synthesized and tested for cyclohexane oxidation. Feactors contributing to the enhanced catalytic activity are identified. For comparison purposes, Au/graphene was synthesized and tested for cyclohexane oxidation to determine the effect of graphene's higher conductivity and large surface area (S. Gilje, S. Han, M. Wang, K. L. Wang, R. B. Kaner, Nano Lett. 7 (2007) 3394-3398; R. Pasricha, S. Gupta, A. K. Srivastava, Small 5 (2009) 2253-2259—each incorporated herein by reference in its entirety) on the activity performance.